Radio Frequency Identification (RFID) systems are commonly used to locate and track items in a near-field communication network including a reader device and at least one wireless terminal, or tag. Energized time-varying electromagnetic radio frequency (RF) waves, which comprise the carrier signal, are transmitted from the reader to the tags in a given RFID network or system. Inductive coupling may be used to transfer energy from one circuit (such as a conductive antenna coil and associated circuitry) to another by means of mutual inductance between the two circuits. A voltage is induced in the tag that can be rectified and used to power the tag circuitry. RFID networks may include tags and readers which exchange information using such inductive coupling between their inductive coupling coils (or antenna coils). To enable data to be passed from the tag to the reader, the tag circuitry changes the load, which is referred to herein as the coupled impedance, associated with its inductive coupling coil. This change can be detected by the reader as a result of the mutual inductive coupling, whereby a reader-originated RF signal can be modified back to the reader, the modified signal being modulated by the tag to transmit encoded data.
FIG. 1 depicts a prior art RFID system in which data transmission from tags 101a-c to reader device 103 is performed on a same frequency channel or spectrum 104. Using the established inductive coupling technology, each of the plurality of tags typically in the RFID system or network sends RF signals on the same carrier signal used for inductive coupling. Hence, the inductive coupling RF signals from each tag overlap those of other tags within the same RF spectrum associated with that given reader device/ RFID network.
As a consequence, tag collision in RFID systems occur when the multiple tags are energized by the same RFID reader device, and simultaneously couple their respective, overlapping signals back to the reader using the given frequency channel. Thus the tag collision problem is exacerbated whenever a large number of tags must be read together in the same RF field. The reader is unable to differentiate these signals when the simultaneously generated signals collide. The tag collisions confuse the reader, generate data transmission errors, and generally reduce data throughput within the RFID system or network.
Various systems have been proposed to isolate individual tags. For example, in one technique aimed at reducing collision errors, when the reader recognizes that tag collision has taken place, it sends a special “gap pulse” signal. Upon receiving this signal, each tag consults a random number counter to determine the interval to wait before sending its data. Since each tag gets a unique number interval, the tags send their data at different times. The adverse impact on overall RFID system performance, in terms of data throughput rate, however, still exists.
Modulating the signal received by the tag and inductively coupling the modulated signal to the reader device is known, using such signal modulation schemes as phase shift keying (PSK) and amplitude shift keying (ASK), where the tag changes its coupled impedance by changing the impedance match between states. However, the adverse effects of tag collisions resulting from overlapping modified signals on a given frequency channel still remain.